Between 20 and 30 employees remain unaccounted for following an explosion at Aeon Mall Kumamoto in Japan's Kumamoto prefecture on Tuesday, after a powerful earthquake struck the region earlier, public broadcaster NHK reported.

It also reported that more than 50 injured people were transported to a hospital in Kumamoto's Minami Ward, with one person unconscious. 

Rescue operations are underway as police and firefighters are checking whether the missing employees are trapped inside the damaged building and searching the site for anyone else who may be trapped, the report adds.

Most customers had already been evacuated to outdoor areas following the explosion that occurred at around 6 p.m. local time, according to local authorities. However, it remains unclear whether any customers are among the missing, the report said.

A total of 3,600 Self-Defense Force personnel have also joined the quake rescue operations, according to Japan's Minister of Defense Shinjiro Koizumi.

A magnitude-6.8 earthquake hit Kyushu, Japan, at a depth of 10 kilometers on Tuesday afternoon, according to the China Earthquake Networks Center.

The Japan Meteorological Agency (JMA) measured the quake at magnitude 7.1, saying it struck Kumamoto Prefecture in southwestern Japan at 4:27 p.m. local time.

Read more: Powerful earthquake hits Japan's Kyushu, tsunami advisory lifted

(With input from Xinhua)